The present invention relates to a drive apparatus for driving a charge coupled device (CCD) solid state image sensor, and, more particularly, to the stable operation of a CCD image sensor and cost reduction for the drive apparatus.
A frame transfer type CCD image sensor, which has the following advantages, is used in cameras or the like.
(1) The efficiency of using light is enhanced.
(2) It is easy to increase the pixel density (to increase the resolution).
The CCD image sensor includes an image sensing section, which performs photoelectric conversion, and a storage section, which is arranged separate from the image sensing section and temporarily stores charges acquired by the photoelectric conversion.
A frame transfer type CCD image sensor further includes a horizontal transfer section which outputs the charges stored in the storage section. The CCD image sensor repeats (1) vertical transfer for transferring the charges acquired by photoelectric conversion to the storage section from the image sensing section at a predetermined time period and (2) horizontal transfer for transferring the charges stored in the storage section to an output section row by row at a high speed.
A drive apparatus, which drives a frame transfer type CCD image sensor, includes a vertical driver, which applies a vertical transfer pulse signal to the image sensing section and storage section, and a horizontal driver, which applies a horizontal transfer pulse signal to the storage section and horizontal transfer section.
The drive apparatus further includes a timing control circuit which generates a timing clock signal in accordance with a system clock signal and provides the timing clock signal to the vertical and horizontal drivers. The timing clock signal determines the timing of generating pulse signals which are output from the vertical and horizontal drivers.
The pulse voltages of the pulse signals, which are output from the vertical and horizontal drivers, are set to a system voltage or a voltage which is acquired by stepping the system voltage up or down by using a power supply circuit.
In the frame transfer type CCD image sensor, applying the vertical transfer pulse signal to the image sensing section causes charges, which are acquired by photoelectric conversion, to be vertically transferred to the storage section from the image sensing section, and applying the horizontal transfer pulse signal to the horizontal transfer section causes horizontal transfer of the stored charges from the horizontal transfer section.
The vertical transfer of charges to the storage section requires a high voltage. Even with a required voltage supplied to the vertical driver from the power supply circuit, therefore, the voltage supplied from the power supply circuit temporarily drops at the time of vertical transfer. It takes a certain time for the dropped voltage to return to the desired voltage level. This affects the vertical transfer operation of the image sensing section.
This problem occurs in an interline type CCD image sensor as well as a frame transfer type CCD image sensor. The interline type CCD image sensor includes an image sensing stage, which performs photoelectric conversion, and a transfer stage, which is coupled in parallel to the image sensing stage and transfers charges, which are acquired by the photoelectric conversion, to a horizontal transfer section. The charge transfer by the transfer stage requires a high voltage, and a voltage supplied to an apparatus for driving the transfer stage also temporarily drops.
A drive circuit, which includes a vertical driver and a horizontal driver, and a power supply circuit are formed separately on a semiconductor integrated circuit substrate. This design is disadvantageous in reducing the yield of a drive apparatus, which includes the drive circuit and the power supply circuit, and cost increase of the drive apparatus. Further, the drive circuit and the power supply circuit, which are provided separately, should face restrictions on mounting on the substrate. This stands in the way of making a CCD-image-sensor drive apparatus compact.
FIG. 1 is a schematic circuit diagram of a buffer circuit 600 which is used in a CCD-image-sensor drive apparatus. The buffer circuit 600 includes an input inverter circuit 3 and an output CMOS inverter circuit 10. The inverter circuit 10 includes a P channel MOS (PMOS) transistor TRp and an N channel MOS (NMOS) transistor TRn connected in series. The source S of the PMOS transistor TRp is connected to a system supply voltage VDD and the source S of the NMOS transistor TRn is connected to ground GND.
An input signal IN, which is supplied to an input terminal 1, is output from an output terminal 2 with a delay of a predetermined time determined by the operational delays of the input inverter circuit 3 and the output CMOS inverter circuit 10. When the buffer circuit 600 is used as an output buffer, the transistors TRp and TRn are designed to have sizes (current capacities) according to a load which is connected to the output terminal 2.
Even with a simple circuit structure, the buffer circuit 600 can delay signals and drive a load. When the transistors TRp and TRn of the output CMOS inverter circuit 10 perform switching actions, however, a non-negligible through current flows through the transistors.
In a load-driving buffer circuit, particularly, the through current of the output CMOS inverter circuit is greater than the through current of a CMOS inverter circuit which is simply used in logic inversion. This inevitably increases the power consumption of a driver which uses a plurality of buffer circuits each having an output CMOS inverter circuit.